Sub‐Auroral Ion Drifts (SAID) Developed Over the Northern Winter Hemisphere at Dawn During 2016–2017

Abstract

We investigated the rare events of sunward (westward) sub‐auroral ion drifts (SAID) development in the dawn magnetic local time (MLT) sector by utilizing horizontal ion velocity measurements collected by the Defense Meteorological Satellite Program F16 spacecraft. We surveyed the 2‐year time period of 2016–2017 and found only 15 SAID detections made in the 3–6 MLT sector over the northern winter hemisphere: seven in 2016 and eight in 2017. Results show that the dawnside SAID developed poleward of the ring‐current‐related plasma density trough, called ring ionospheric trough (RIT). With a few inner‐magnetosphere observations, we demonstrated that the favorable conditions were created by (a) the proton/ring current injections occurring across the newly‐formed dawnside plasmapause where (b) the ring current ions became unstable. As a result of (a), the outward SAID electric (E) field developed via short‐circuiting, mapped down to the ionosphere as a poleward E field and drove the sunward (westward) SAID flow in the dawn MLT sector. As a result of (b), a localized heat source developed near the newly‐formed dawnside plasmapause in the hot zone and the resultant downward heat flux led to stable auroral red (SAR) arc development when the electron temperature was sufficiently high. Then, the dawnside SAID flow developed parallel with the RIT and SAR arc.